This invention relates generally to organic light emitting device (OLED) displays that have light emitting layers.
OLED displays use layers of light emitting polymers or short molecule materials. Unlike liquid crystal devices, the OLED displays actually emit light making them advantageous for many applications.
Some OLED displays use at least one semiconductive conjugated polymer sandwiched between a pair of contact layers. Other OLED displays use small molecules. The contact layers produce an electric field that injects charge carriers into the light emitting layer. When the charge carriers combine in the light emitting layer, the charge carriers decay and emit radiation in the visible range.
It is believed that polymer compounds containing vinyl groups tend to degrade over time and use due to oxidation of the vinyl groups, particularly in the presence of free electrons. Since driving the display with a current provides the free electrons in abundance, the lifetime of the display is a function of total output light. Newer compounds based on fluorine have similar degradation mechanisms that may be related to chemical purity, although the exact mechanism is not yet well known in the industry. In general, OLED displays have a lifetime limit related to the total output light. This lifetime is a function of the display usage model.
The OLED display can be driven so as to increase its useful lifetime because as the display degrades, its output light is decreased. One way to drive the display to increase lifetime is to drive the display to increase the display's brightness. However, degradation may introduce output non-uniformity errors. If some of the pixels of the display are degraded non-uniformly, simply increasing the drive current of the display does not solve the non-uniform degradation problem. Even after increasing the drive current, some pixels will be brighter than other pixels.
Thus, there is a continuing need for ways of controlling OLED displays that compensate for display aging.